This invention relates generally to oil and gas well logging tools. More particularly, this invention relates tools for measuring rock formation density through the use of gamma rays generated by a pulsed neutron source. This invention relates to an improved density tool that may be used in cased holes as well as open holes.
In petroleum and hydrocarbon production, it is desirable to know the porosity of the subterranean formation which contains the hydrocarbon reserves. Knowledge of porosity is essential in calculating the oil saturation and thus the volume of oil in-place within the reservoir. Knowledge of porosity is particularly useful in older oil wells where porosity information is either insufficient or nonexistent to determine the remaining in-place oil and to determine whether sufficient oil exists to justify applying enhanced recovery methods. Porosity information is also helpful in identifying up-hole gas zones and differentiating between low porosity liquid and gas.
If the density of the formation is known, then porosity can be determined using known equations. A variety of tools exist which allow the density of the reservoir to be determined. Most of these tools are effective in determining the density (and hence porosity) of the reservoir when the wellbore in which the tool is run is an uncased reservoir and the tool is able to contact the subterranean medium itself. However, once a well has been cased, there exists a layer of steel and concrete between the interior of the wellbore where the tool is located and the formation itself. The well casing makes it difficult for signals to pass between the tool and the reservoir and vice-versa. In addition, the cement can confuse the measurement of formation properties.
Devices have been proposed which would use a chemical radioactive source to generate a response signal, similar to the open-hole counterparts, which are commercially available. A chemical radioactive source tool would have a limited response due to the more complex borehole environment that generally exists in cased wells, and parameters such as the depth-of-investigation would be limited. The count rates would also be low due to the impedance introduced by the well casing. Increasing the strength of the radioactive source would not be desirable because of the safety concerns of using a strong radioactive source. Contamination problems also become a concern when using strong radioactive sources. Additionally, a large number of cased wells contain tubing within the casing. Because of the inability to contact the tool with the side of the wellbore in a wellbore containing tubing, even the chemical radioactive source tools would not work in this situation since the emitted particles will seek a path of low density and therefore migrate into the annulus between the side of the wellbore and the tubing.
A different approach involves detection of gamma radiation produced in the formation in response to a high-energy neutron source, referred to as induced gamma ray logging. When the neutron source is pulsed, gamma rays are produced by one of two reactions. The first is inelastic scattering of fast neutrons (neutrons with energies above of about one MeV or within about one order of magnitude). The second mechanism is from capture of epithermal neutrons (neutrons with an energy of about one eV). The third is from capture of thermal neutrons (neutrons with an energy of about 0.025 eV). The fast-neutron lifetimes are very small (a few microseconds) such that during the source pulse a mixed-energy neutron field exists. Shortly after the burst, all neutrons slow down to a thermal energy level and these thermal neutrons wander about until being captured, with a lifetime in the hundreds of microseconds. Gamma rays from inelastic scattering are produced in close proximity to the accelerator, and gamma rays from thermal capture are dispersed farther from the accelerator (up to tens of centimeters). The number of capture gamma rays is strongly influenced by the amount of hydrogen and the thermal neutron capture cross section of the formation. The number of gamma rays produced from inelastic scattering is less dependent on these quantities, and a measurement of such gamma rays is more directly related to the formation density. Use of a pulsed neutron source allows capture gamma rays to be separated from inelastic gamma rays, giving a better estimate of density. Examples of pulsed neutron sources are given in U.S. Pat. No. 5,900,627 to Odom et al. and U.S. Pat. No. 5,825,024 to Badruzzaman.
Formation density measurements have traditionally been made using two gamma ray detectors. In open hole situations, density estimates ρSS and ρLS made by the near and far detectors are used to get a corrected density estimate using the spine and rib method which may be represented by the equationρ−ρLS=Δρ=ƒ(ρLS−ρSS)  (1),where ƒ(·) is a function that is nonlinear, depends upon the standoff of the tool or the amount of mud cake between the tool and formation, and determined by a calibration process. This dual detector arrangement is able to compensate for standoff (in MWD applications) and mudcake thickness (in wireline applications). When used with a pulsed neutron source, correction also has to be made for variations in the source intensity, so that a two detector arrangement only gives a single estimate of density based on, for example, a ratio of the outputs of the two detectors.
For measurements made in cased holes, there is an additional complication due to the presence of casing and cement. In order to probe the formation, neutrons must exit the tool, pass through the casing and cement and scatter, or be captured in the formation before the resulting gamma rays passing back through the cement and the casing to finally reenter the tool to be detected. Thus, instead of just a mudcake coffection (for open hole wireline) or a standoff correction (for MWD), a cased hole density tool must be able to correct or compensate for the cement and casing, an effect which is greater than that of the mudcake. U.S. Pat. No. 5,525,797 to Moake discloses the use of a three detector tool using a chemical gamma ray source which coffects for the effects of casing. A drawback of the Moake device is the need for a relatively high energy chemical source (a safety issue), and the fact that gamma ray intensities are measured (instead of count rates).
Badruszaman discloses an embodiment in which four detectors are used in combination with a pulsed neutron source. This would, in principle, be able to compensate for both source variations, the effect of casing and the effects of mudcake. However, the teachings of Badruzzaman are lacking in exactly how the density is determined. The present invention addresses this deficiency.